Self-assembled monolayer-functionalized half-metallic manganite for molecular spintronics

Multifunctional Surface Treatment against Imperfections and Halide Segregation in Wide-Band Gap Perovskite Solar Cells

Mixed-halide wide-band gap perovskites (WBPs) still suffer from losses due to imperfections within the absorber and the segregation of halide ions under external stimuli. Herein, we design a multifunctional passivator (MFP) by mixing bromide salt, formamidinium bromide (FABr) with a p-type self-assembled monolayer (SAM) to target the nonradiative recombination pathways. Photoluminescence measurement shows considerable suppression of nonradiative recombination rates after treatment with FABr. However, WBPs still remained susceptible to halide segregation for which the addition of 25% p-type SAM was effective to decelerate segregation. It is observed that FABr can act as a passivating agent of the donor impurities, shifting the Fermi-level (Ef) toward the mid-band gap, while p-type SAM could cause an overweight of Ef toward the valence band. Favorable band bending at the interface could prevent the funneling of carriers toward I-rich clusters. Instead, charge carriers funnel toward an integrated SAM, preventing the accumulation of polaron-induced strain on the lattice. Consequently, n-i-p structured devices with an optimal MFP treatment show an average open-circuit voltage (VOC) increase of about 20 mV and fill factor (FF) increase by 4% compared with the control samples. The unencapsulated devices retained 95% of their initial performance when stored at room temperature under 40% relative humidity for 2800 h.

Combined spin filtering actions in hybrid magnetic junctions based on organic chains covalently attached to graphene

We present a bias-controlled spin-filtering mechanism in spin-valves including a hybrid organic chain/graphene interface. Wet growth conditions of oligomeric molecular chains would usually lead, during standard CMOS-compatible fabrication processes, to the quenching of spintronics properties of metallic spin sources due to oxidation. We demonstrate by X-ray photoelectron spectroscopy that the use of a protective graphene layer fully preserves the metallic character of the ferromagnetic surface and thus its capability to deliver spin polarized currents. We focus here on a small aromatic chain of controllable lengths, formed by nitrobenzene monomers and derived from the commercial 4-nitrobenzene diazonium tetrafluoroborate, covalently attached to the graphene passivated spin sources thanks to electroreduction. A unique bias dependent switch of the spin signal is then observed in complete spin valve devices, from minority to majority spin carriers filtering. First-principles calculations are used to highlight the key role played by the spin-dependent hybridization of electronic states present at the different interfaces. Our work is a first step towards the exploration of spin transport using different functional molecular chains. It opens the perspective of atomic tailoring of magnetic junction devices towards spin and quantum transport control, thanks to the flexibility of ambient electrochemical surface functionalization processes.

Conductance switching at the nanoscale of diarylethene derivative self-assembled monolayers on La0.7Sr0.3MnO3

We report on the phosphonic acid route for the grafting of functional molecules, optical switch (dithienylethene diphosphonic acid, DDA), on La_0.7Sr_0.3MnO₃ (LSMO). Compact self-assembled monolayers (SAMs) of DDA are formed on LSMO as studied by topographic atomic force microscopy (AFM), ellipsometry, water contact angle measurements and X-ray photoemission spectroscopy (XPS). The conducting AFM measurements show that the electrical conductance of LSMO/DDA is about 3 decades below that of a bare LSMO substrate. Moreover, the presence of the DDA SAM suppresses the known conductance switching of the LSMO substrate that is induced by mechanical and/or bias constraints during C-AFM measurements. A partial light-induced conductance switching between the open and closed forms of the DDA is observed for the LSMO/DDA/C-AFM tip molecular junctions (closed/open conductance ratio of about 8). We show that, in the case of long-time exposure to UV light, this feature can be masked by a non-reversible decrease (a factor of about 15) of the conductance of the LSMO electrode.

Spin Selectivity in Photoinduced Charge-Transfer Mediated by Chiral Molecules

Optical control and readout of electron spin and spin currents in thin films and nanostructures have remained attractive yet challenging goals for emerging technologies designed for applications in information processing and storage. Recent advances in room-temperature spin polarization using nanometric chiral molecular assemblies suggest that chemically modified surfaces or interfaces can be used for optical spin conversion by exploiting photoinduced charge separation and injection from well-coupled organic chromophores or quantum dots. Using light to drive photoexcited charge-transfer processes mediated by molecules with central or helical chirality enables indirect measurements of spin polarization attributed to the chiral-induced spin selectivity effect and of the efficiency of spin-dependent electron transfer relative to competitive relaxation pathways. Herein, we highlight recent approaches used to detect and to analyze spin selectivity in photoinduced charge transfer including spin-transfer torque for local magnetization, nanoscale charge separation and polarization, and soft ferromagnetic substrate magnetization- and chirality-dependent photoluminescence. Building on these methods through systematic investigation of molecular and environmental parameters that influence spin filtering should elucidate means to manipulate electron spins and photoexcited states for room-temperature optoelectronic and photospintronic applications.

Functional Organophosphonate Interfaces for Nanotechnology: A Review

Optimization of interfaces in inorganic-organic device systems depends strongly on understanding both the molecular processes that are involved in surface modification and the effects that such modifications have on the electronic states of the material. In particular, the last several years have seen passivation and functionalization of semiconductor surfaces to be strategies by which to realize devices with superior function by controlling Fermi level energies, band-gap magnitudes, and work functions of semiconducting substrates. Among all of the synthetic routes and deposition methods available for the optimization of functional interfaces in hybrid systems, organophosphonate chemistry has been found to be a powerful tool to control at the molecular level the properties of materials in many different applications. In this Review, we focus on the relevance of organophosphonate chemistry in nanotechnology, giving an overview about some recent advances in surface modification, interface engineering, nanostructure optimization, and biointegration.

Activating the molecular spinterface

The miniaturization trend in the semiconductor industry has led to the understanding that interfacial properties are crucial for device behaviour. Spintronics has not been alien to this trend, and phenomena such as preferential spin tunnelling, the spin-to-charge conversion due to the Rashba-Edelstein effect and the spin-momentum locking at the surface of topological insulators have arisen mainly from emergent interfacial properties, rather than the bulk of the constituent materials. In this Perspective we explore inorganic/molecular interfaces by looking closely at both sides of the interface. We describe recent developments and discuss the interface as an ideal platform for creating new spin effects. Finally, we outline possible technologies that can be generated thanks to the unique active tunability of molecular spinterfaces.

Fabrication of ultra-smooth and oxide-free molecule-ferromagnetic metal interfaces for applications in molecular electronics under ordinary laboratory conditions

Direct self-assembly of n-alkanethiolate SAMs on ferromagnetic metal surface was fabricated. The stability and tunnelling characteristics of SAMs were investigated.

Engineering the magnetic coupling and anisotropy at the molecule-magnetic surface interface in molecular spintronic devices

A challenge in molecular spintronics is to control the magnetic coupling between magnetic molecules and magnetic electrodes to build efficient devices. Here we show that the nature of the magnetic ion of anchored metal complexes highly impacts the exchange coupling of the molecules with magnetic substrates. Surface anchoring alters the magnetic anisotropy of the cobalt(II)-containing complex (Co(Pyipa)₂), and results in blocking of its magnetization due to the presence of a magnetic hysteresis loop. In contrast, no hysteresis loop is observed in the isostructural nickel(II)-containing complex (Ni(Pyipa)₂). Through XMCD experiments and theoretical calculations we find that Co(Pyipa)₂ is strongly ferromagnetically coupled to the surface, while Ni(Pyipa)₂ is either not coupled or weakly antiferromagnetically coupled to the substrate. These results highlight the importance of the synergistic effect that the electronic structure of a metal ion and the organic ligands has on the exchange interaction and anisotropy occurring at the molecule–electrode interface.

Controlling the magnetic response of a molecular device is important for spintronic applications. Here the authors report the self-assembly, magnetic coupling, and anisotropy of two transition metal complexes bound to a ferrimagnetic surface, and probe the role of the nature of the transition metal ion.

Self-assembled monolayers based spintronics: from ferromagnetic surface functionalization to spin-dependent transport

Chemically functionalized surfaces are studied for a wide range of applications going from medicine to electronics. Whereas non-magnetic surfaces have been widely studied, functionalization of magnetic surfaces is much less common and has almost never been used for spintronics applications. In this article we present the functionalization of La2/3Sr1/3MnO3, a ferromagnetic oxide, with self-assembled monolayers for spintronics. La2/3Sr1/3MnO3 is the prototypical half-metallic manganite used in spintronics studies. First, we show that La2/3Sr1/3MnO3 can be functionalized by alkylphosphonic acid molecules. We then emphasize the use of these functionalized surfaces in spintronics devices such as magnetic tunnel junctions fabricated using a nano-indentation based lithography technique. The observed exponential increase of tunnel resistance as a function of alkyl chain length is a direct proof of the successful connection of molecules to ferromagnetic electrodes. For all alkyl chains studied we obtain stable and robust tunnel magnetoresistance, with effects ranging from a few tens to 10 000%. These results show that functionalized electrodes can be integrated in spintronics devices and open the door to a molecular engineering of spintronics.

Electrochemical growth of CoNi and Pt-CoNi soft magnetic composites on an alkanethiol monolayer-modified ITO substrate

CoNi and Pt-CoNi magnetic layers on indium-tin oxide (ITO) substrates modified by an alkanethiol self-assembled monolayer (SAM) have been electrochemically obtained as an initial stage to prepare semiconducting layer-SAM-magnetic layer hybrid structures. The best conditions to obtain the maximum compactness of adsorbed layers of dodecanethiol (C12-SH) on ITO substrate have been studied using contact angle, AFM, XPS and electrochemical tests. The electrochemical characterization (electrochemical probe or voltammetric response in blank solutions) is fundamental to ensure the maximum blocking of the substrate. Although the electrodeposition process on the SAM-modified ITO substrate is very slow if the blocking of the surface is significant, non-cracked metallic layers of CoNi, with or without a previously electrodeposited seed-layer of platinum, have been obtained by optimizing the deposition potentials. Initial nucleation is expected to take place at the pinhole defects of the C12-SH SAM, followed by a mushroom-like growth regime through the SAM interface that allows the formation of a continuous metallic layer electrically connected to the ITO surface. Due to the potential of the methodology, the preparation of patterned metallic deposits on ITO substrate using SAMs with different coverage as templates is feasible.

Magnetic behaviour of TbPc2 single-molecule magnets chemically grafted on silicon surface

Single-molecule magnets (SMMs) are among the most promising molecular systems for the development of novel molecular electronics based on the spin transport. Going beyond the investigations focused on physisorbed SMMs, in this work the robust grafting of Terbium(III) bis(phthalocyaninato) complexes to silicon surface from a diluted solution is achieved by rational chemical design yielding the formation of a partially oriented monolayer on the conducting substrate. Here, by exploiting the surface sensitivity of X-ray circular magnetic dichroism we evidence an enhancement of the magnetic bistability of this single-molecule magnet, in contrast to the dramatic reduction of the magnetic hysteresis that characterises monolayer deposits evaporated on noble and ferromagnetic metals. Photoelectron spectroscopy investigations and density functional theory analysis suggest a non-innocent role played by the silicon substrate, evidencing the potentiality of this approach for robust integration of bistable magnetic molecules in electronic devices.